Printing presses use plates to print ink onto paper and other media. One method used for creating plates has been to expose photosensitive film with the matter to be printed. When the film is developed, the matter imaged on the film is imaged onto a photosensitive plate, sometimes referred to as "burning" a plate. After processing, the plate can be used to print the matter onto a medium.
In a black and white printing job, there is usually one plate that is used to print black ink. In a color printing job, a different plate is used for each color ink. Typically, a color job will use three colors of ink: cyan, magenta, and yellow. This is because a combination of cyan, magenta, and yellow can be used to make other colors. A plate is produced for each color ink. Often, in addition to cyan, magenta, and yellow, black ink is also used. An additional plate is then required to print the black ink. Occasionally, one or more colors will be printed separately as well, referred to as a "spot color." That color will also have its own plate.
Electronic prepress systems have used an imagesetter to receive raster data for imaging onto photosensitive film. The film is then used to create a plate. The imagesetter exposes the photosensitive film pixel by pixel, for instance, by scanning a laser across and down a piece of film. Electronics controls the laser to expose, or refrain from exposing, each pixel in the raster data in a precise and repeatable manner. Recently, platesetters also have been used to create plates directly from raster data without the use of film. Imagesetters, platesetters and other output devices for printing are generally referred to as print engines.
Print engines typically have been served by a dedicated raster image processor (RIP) connected between the print engine and a "front end" computer running imaging application software such as Quark Express.TM. and Adobe Pagemaker.TM.. Exemplary front end computers run on operating systems such as Windows NT.TM., MacOS.TM. and UNIX.TM.. In a typical configuration, a Macintosh.TM. front end is connected to a RIP which is coupled with an imagesetter. The RIP interprets the graphic information transmitted to it by the front end computer, and converts the graphic information into raster data that can be imaged by the print engine. The raster data produced by the RIP is configured to match required parameters of both the imagesetter and the media The imagesetter parameters include imaging resolution, processing speed and specific printing capabilities. The media parameters include the length, width and thickness of the media, as well as the chemical makeup of the photosensitive layer.
Typically, the imaging application software provides output in the format of a page description language (PDL) such as Postscript.TM. and PDF.TM. offered by Adobe Systems of Mountain View, Calif. Page description languages describe images using descriptions of the objects contained in the page. Use of page description languages allows pages to be described in a way that can be interpreted appropriately for imaging at various sizes and resolutions. PDL code is generally significantly smaller in data size than the raster data that results from interpreting the PDL code. Use of a page description language therefore allows for faster file transfer. Also, page description languages are machine-independent so that any print engine or other device which understands the PDL can produce an image therefrom.
When PDL image data is received by the RIP, operations performed by the RIP, such as using fonts to lay out text and color processing to create raster data for each color, typically results in one or more raster data bit maps. The raster data produced by the RIP is binary, meaning that each pixel is either on or off. The raster data for each of the colors in a color image is referred to as a color separation.
Each color separation is transferred from the RIP to the output device over a high speed interface. This has historically been a parallel data transfer interface that provides a data transfer rate sufficient to keep the output device operating at a desired operating speed. Typically, the process of RIP processing data to prepare bit map image files for transfer to the output device has been slower than the imaging speed of the output devices. The slower RIP processing speed sometimes causes the output device to remain idle while waiting for a RIP to prepare the next bit map image file. The print engine is generally an expensive capital investment, so full time utilization of the print engine is desirable. Keeping the print engine busy is therefore a goal of modern electronic prepress system design.
The use of a RIP multiplexer (MUX), for example the MULTISTAR.RTM. offered by Agfa Division of Bayer Corporation of Wilmington, Mass., offers the electronic prepress industry some improvement in data throughput, and associated cost savings, by functioning as a data buffer between one or more RIPs and a print engine. Cost savings and improved efficiency have been realized by either RIP processing an image with a first RIP while transferring a previously RIP processed image to the output device or by storing RIP processed raster data for transfer to the output device at an appropriate time after RIP processing. This multiplexer more fully utilizes the output device, and therefore provides increased throughput.
Typically, for prior art electronic prepress systems, a specific output device configuration had to be connected to the RIP before a job could be processed. For example, a print job requiring that a particular type of imagesetter be used for an output device, or that a particular media type or size be loaded onto the output device, could not be RIP processed into raster data if the particular output device connected to the RIP did not meet the job requirements. Improper output device configuration caused delay or, more frequently, required that a user take some action to physically change the output device connected to the RIP in order to continue processing and outputting image files. Since the electronic and imagesetting systems of the prior art were not only device dependent but media dependent as well, the queuing of rasterized print jobs for different media or output devices was not possible. Thus, the choice of the output device and print media proved to be a considerable hindrance in productivity.
RIP processing speed has improved so that the RIP is no longer a bottleneck in the prepress workflow of single page printing jobs. As RIP processing speed has increased, however, so has the demands of output devices. Recent use of larger format imagesetters and platesetters allows multi-page press size images in film or plate, referred to as "flats," to be produced that contain four, eight, or more pages in each image. These output devices also have been driven by a dedicated RIP or MUX. Because multi-page flats are complex, the RIP is often a bottleneck in creating these multi-page press format films and plates. The PDL code that must be interpreted to image multiple page flats is very complex. RIP processing time for complex images can require several multiples of the imaging time.
RIP processing time has a greater impact on workflow when a change is required in a complex image. This is because a change in even a part of one page of a multi-page flat generally requires that the entire image be reprocessed by the RIP. The bottleneck of slow RIP speeds for complex images affects the workflow both the first time the flat is processed by the RIP and the second time when a modified version of the image is processed.
One alternative to reprocessing the entire image, when a modification to a RIP processed image is desired, is to physically modify a film that is output by an imagesetter to make a plate. To accomplish this modification, a portion of the image to be modified is physically cut from the film, and if necessary, a correction film is inserted in its place. This can be difficult to accomplish without imaging artifacts. More importantly, this alternative is not possible with direct-to-plate, i.e. computer-to-plate, technology.
Another technique to modify images once they have been processed by the RIP is known as doubleburning. To conventionally doubleburn an image onto a plate is to create two pieces of film and create the plate from both of the images. In other words, the photosensitive plate is physically exposed to two pieces of film, and the resulting plate includes the images from both pieces of film. This is particularly useful for producing composite images where one part of the image has several possible versions. An example of such a composite image contains graphics and text, with different versions of text to be imaged with the same graphics. It can be time consuming to reprocess the complex graphics with each of the variations of text. One technique to reduce RIP processing time is to image graphics once onto a film, leaving a blank space where the particular variation of text is to be inserted into the image, then imaging the text separately onto a second film, without the need to again image the graphics. A plate having both the graphics and one version of the text is produced by burning the plate twice, once with the film containing only the graphics, and once with the film containing only the text. This doubleburning technique is unavailable in direct-to-plate technology since film is not used in the direct-to-plate process of making plates.